1. Field of the Invention
The present invention relates to a bearing assembly and in particular to a bearing assembly including rolling bearing elements such as balls or rollers.
The invention has been developed primarily for use in carriage assemblies, known as “travellers”, disposed to traverse guide rails on sailing craft. It will therefore be described primarily with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.
2. Description of the Prior Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Travellers have been used in yacht fittings for many years. These devices typically incorporate uncaged ball or roller bearing assemblies to minimise friction as they run along a complementary guide rail on which they are slidably but captively retained. These bearing assemblies typically include either polyamide-imide or acetal bearing elements and run on anodised aluminum rails, relying on water for lubrication. Bearing elements formed of polyamide-imide or acetal have a high surface resistivity of around 1018 ohm, as measured using ASTM D257. Accordingly, when dry, they retain static charge and strongly attract dirt and dust particles.
The bearing assemblies are not sealed and accordingly the bearing elements are exposed to contamination by dirt, grit and salt, all of which adhere strongly to the surfaces of the elements, causing increased friction. It will be appreciated that under these conditions, the use of hydrocarbon lubricants exacerbates the problem of dirt and grit accumulation on surfaces of the rails and bearing elements, as the dirt and grit tends to adhere to the hydrocarbon lubricant.
Sometimes the rail anodising is treated with a polytetrafluoroethylene (“PTFE”) coating. This coating can be applied using one of two methods, both of which have detrimental consequences. The first method involves high temperature fusing, which substantially reduces the yield strength of the aluminium in the rails, rendering it unsuitable for high stress applications. The second method involves applying a varnish-type suspension to the rail anodising. However, the varnish has a short service life of about twelve months, after which time the coating must be reapplied.
In use, the bearing elements rub against each other at double the translational speed of the traveller, causing wear and subsequently increased friction. As a result, the balls eventually skid, which in turn wears flats onto the bearing surfaces. This severely compromises both the efficiency and performance of the traveller. Another problem occurs when these bearing assemblies operate in dry conditions, for example on the mast. In these conditions, the rail tends to collect dust, which adheres to the bearing elements, rendering them dull and again increasing friction.
It is an object of the invention, in its presently contemplated form, to overcome or ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.